Adding Material to Cycles Material library in Material Library VX addon
Got it working. It is possible. But a bit tricky. The filter category has to be off. Then it works. The steps are:

Turn filter categories off
Save Blend file with your new material.
Insert selected Material
Select material in the addon list
Switch to the category where you want to insert it.
Click at arrow button
Turn on filter categories
Refresh Library by clicking at the Filter Category button.